The invention pertains to the field of fluid valves. More particularly, the invention pertains to cushioned relief valves.
Water hammer, also known as hydraulic shock, can occur in piping systems that carry a high momentum fluid when rapid changes in momentum of the fluid take place, for example, when a valve or pump in the system is abruptly closed and the fluid stops flowing. As flowing fluids generally have a constant density and mass, changes in fluid momentum result from, for example, changes in fluid flow velocity, a cessation of fluid flow, or a reversal of flow direction causing retrograde flow. When a valve or pump in the system is closed or shut off and fluid flow within the system suddenly stops, the change in fluid flow velocity causes a shockwave to form and propagate through the fluid and piping structures that carry the fluid. The shockwave may be characterized, physically and mathematically, as a transient high pressure pulse moving through the fluid flow system 20.
When the shockwave impacts valve gates, pumps and other solid structures, the energy carried by the high pressure of the shockwave is transferred to these solid structures. The shockwave pressure impacting piping, pump and valve structures is undesirable, as it is a source of unwanted acoustic noise, vibration, and extreme pressure gradients that may cause significant mechanical stress on pipes, pumps, valves, and other fixtures. In extreme cases, pipes may burst from excessive pressure extremes associated with a shockwave, or conversely may implode at a location as a result of shockwave formation at another location.
In some systems, for example lift stations bringing a fluid such as water or sewage from one elevation to a higher elevation, check valves are often used to prevent or retard retrograde fluid flow when pumping systems are turned off or valves are closed. Changes in pump status when a pump is turned off, and closing of the check valve in these systems, may cause significant hydraulic shock, particularly when large diameter pipes and large differences in elevation are involved.
In conventional systems, various solutions to mitigate hydraulic shock have been employed. In some solutions, the fundamental mitigation approach has been to either provide an alternative energy absorbing pathway for fluids to flow in, so that shockwave energy is dissipated when hydraulic shock occurs. In other approaches, the rate at which changes in flow velocity occur is regulated in order to prevent the formation of shockwaves at their source, or minimize the energy and extreme pressure increases associated with shockwaves.
Water towers, or vertical water column shunts, commonly provide alternative energy absorbing pathways. Fluids being pumped from a lower elevation to a higher elevation tend to reverse direction and produce retrograde flow back to the lower elevation when pumps are turned off or valves are closed. An open topped water tower or vertical water column located between the two elevations and at a higher elevation than a check valve, allows the retrograde flow and shockwave energy to be redirected upwardly into the tower or water column, against the force of gravity, thus harmlessly absorbing the shockwave energy and preventing hydraulic shock.
Buffers, such as tanks filled with a compressible gas, may also be incorporated in fluid systems to absorb shockwave energy and pressure, and reduce or eliminate hydraulic shock. Retrograde flow redirected toward the tank increases the fluid pressure in the tank, which in turn compresses the compressible gas, and shockwave energy is thus absorbed and then fed back into the fluid system by the initial compression and subsequent expansion of the gas after the fluid system returns to nominal operating pressures.
In other mitigation approaches, basic considerations such as valve closing rates, pump rate of stop, and length of straight-line piping between elevations may be adjusted to also reduce or eliminate hydraulic shock.
In the case of pump stoppages, hydraulic shock occurs when a pump stops suddenly, causing a sudden change in fluid flow velocity in piping connected to the pump. Adding a massive flywheel to the pump, for example, slows the rate at which pumping stops when power to the pump is removed, and thus slows the rate of change of fluid flow velocity, so that shock waves are not produced, or their pressure amplitude is minimized Alternatively, short continuous straight-line runs of piping between elevations, such as serpentine pathways, may also minimize hydraulic shock. Bends in a pipeline decrease the total mass of fluid flowing together in a section of pipe in a given direction, and therefore also decrease the total momentum of the fluid flowing in that section of pipe.
Since basic system design considerations may not always adjust to mitigate hydraulic shock, or are cost prohibitive, cushioned check valves have been developed that change the rate of check valve closing to mitigate hydraulic shock. In these conventional cushioned check valves, fluid being pumped from a lower elevation to a higher elevation may stop flowing toward the higher elevation, and reverse direction toward the lower elevation as valves are closed, or pumps stop pumping, while a check valve closes.
For example, a check valve in-line in a lift station between a lower elevation and a higher elevation requires a certain amount of time to close when movement of fluid toward the higher elevation stops, and retrograde flow begins to carry a valve disk backward toward a valve seat until the check valve closes and stops the retrograde flow. The fluid being pumped may therefore develop significant retrograde flow velocity toward the lower elevation that causes hydraulic shock with a significant amount of energy and pressure when the valve disk ultimately closes, and the retrograde flow abruptly stops.